Bladder valves of various kinds have been know and used for many years. One form of bladder valve is shown in U.S. Pat. No. 4,268,005 issued May 19, 1981 to Raftis. A pump incorporating bladder valves is shown in U.S. Pat. No. 3,007,416 issued Nov. 7, 1961 to Childs.
It will be apparent that bladder valves are used for a variety of different applications. In the Childs patent the bladder valves are use as a heart pump, however this type of valve is particularly suited for handling material in slurry form such as metallic ores, sand, coal, wood chips, etc. and is most frequently used for these purposes.
It has also been proposed to incorporate bladder valves in a pump for frail material such as a fish pump as shown in U.S. patent application Ser. No. 082,508 filed Aug. 8, 1987 by Breckner (the same inventor as the current application.)
When bladder valves are used particularly with frail material such as fish problems may be encountered with the bladders squeezing the fish and possibly damaging same when the bladders are inflated to close the valve passage and the product (fish) is in the passage as it is being closed. Also the abrasive nature of some materials normally passing through these valves causes wear and thus clearing at least some of this material from the valve before closing can increase valve life.